Production of products from steel slags

ABSTRACT

A method for treatment of metallurgical wastes i.e. slags, which are in liquid form by introducing water into the liquid slag thus causing it to cool at a slow rate so that the solidified slag contains a porous crystalline or utrified structure according to the rate of cooling, which structure is stable at ambient temperature. Another object of the invention concerns an apparatus for treating solid metallurgical slags, comprising a continuous conveyor formed of molds and means adapted to diffuse water into the liquid slag.

The present invention concerns a method for treating solid metallurgicalwaste commonly known as “slags” coming from steelworks furnaces of theLinz Donavitz convertor, electric arc or similar, capable of being usedby converting, for instance, into the composition of cement, aggregateor similar, and devices implementing the method.

In the area of metallurgy, there well known methods involving thetreatments of slags which are recovered at the base of blast furnaces inorder to allow their recycling. The most widespread method, such as themethod described in DE-A-2852993, consists in pouring the molten slaginto a large vessel filled with water in which the slag bursts intopieces by reason of thermal shock. This type of method has the drawbackof requiring a heavy and cumbersome plant in order not only to protectthe operatives from slag explosions but also in order to remove theparticles of solidified slag after they have cooled.

In order to avoid this drawback, dry cooling blast furnace slagtreatment methods have already been devised; see for instance,WO97/43455 describing a method for the continuous production of avitreous or vitrified blast furnace slag. According to this method, theslag is applied on a cooling roller or on to cooling slabs joinedtogether in an articulated manner on an endless conveyor belt andcooling of the slag being effected dry, i.e. without water spraying. Theliquid slag is carried to an intermediate vessel, removed from thisvessel at temperatures higher than 1350° C. into a distribution systemin parallel sections onto a continuous belt supporting the cooling slabsor on a cooling roller, the cooled slag being finally ejected and brokenup in a crusher or grinder.

If the blast furnace slag has a sufficient chemical stability, theparticles of solid slag obtained according to these methods are thengraded according to their grain size into processing for ballast forrailway tracks, into the processing of bitumen, cements etc.

However, in the area of metallurgy, and more particularly in that of thesteelworks, there are well known oxygen lance steelworks furnaces calledLD furnaces, convertor furnaces, electric arc furnaces (E.A.F.),refining furnaces under partial vacuum of the A.O.D. or V.O.D. type orsimilar which produce slags which, after treatment according to themethods such as described previously, have a higher lime content and anunstable chemical structure so that in contrast to blast furnace slags,only a tiny fraction of this slag can be used. As a result slag comingfrom these furnaces is usually poured into open air pits causing therisk of pollution.

One of the objects of the invention is therefore to provide a method fortreating such slags (coming from steelworks furnaces of the LD type,convertor or similar) to provide a structure which is stable at ambienttemperature so as to allow them to be utilised.

To this purpose and according to the invention, one aspect of theinvention there is provided a method for the treatment of steel wastes,commonly called steel slags, which are in a liquid form at a temperaturewhich is higher than 1000° C. on coming out of a steelworks furnace ofthe LD type, convertor, electric arc or similar, and are cooled to atemperature at which the liquid slag starts to solidify, the methodcomprising (i) introducing water into the liquid steel slag and (ii)causing the resultant material to cool at a slow rate so that thesolidified steel slag contains a porous, crystalline or vitrifiedstructure according to the rate of cooling, which structure is stable atambient temperature.

According to a first variant of the method according to the invention,the water is introduced by means of at least one lance, e.g. into themass of liquid slag in such a manner that on contact with the slag thewater converts into water vapour which diffuses into the slag withoutcausing an explosion, then the slag is poured in streams onto a conveyorin order to cool it slowly.

According to another variant of the invention, the water is introducedinto a stream of liquid slag, passing from a tundish in which the slagis collected from the furnace to the convertor where it is cooled.

According to another variant of the invention, the method consists incontinuously pouring a fine granular material on a conveyor formed by acontinuous conveyor, then in spraying water on a fine granular materialso as to moisten it and in pouring continuously the liquid slag on tothe moistened fine granular material, so that the water contained in thefine granular material is converted into water vapour which diffusesinto the slag. The fine granular material consists preferably of anaggregate of metallurgical slag with an average particle size of between0 to 6 mm conveniently known as fines. The water is sprayed onto thefines in such a way that the moisture is retained but does not form acontinuous body of water.

In order to modify the properties of the solid slag obtained byimproving, e.g. the “hydraulicity” of the slag in such a manner that thelatter can enter into the composition of a cement, an additiveconsisting in one or a number of elements—according to the nomenclatureof the cement constituents—Si, Al, Fe, Ca, Cr, O, C, B, K, F, and P isintroduced with the water or into the slag.

Furthermore, the volume of water diffused in the slag is at least equalto the volume of water necessary to transform the hydratable limecontained in the liquid slag from the steelworks furnaces.

Another object of the invention concerns a device for treating solidmetallurgical slags, the which are in a liquid form and which iscollected in a tundish on coming out from the blast furnace or asteelworks furnace of the LD type, convertor, electric arc or similar,being poured continuously and in a stream on a continuous conveyorformed of articulated moulds so as to cool gradually the solidifyingliquid slag, characterised in that comprises means capable of diffusingwater in the liquid slag without causing an explosion.

It will be readily understood that the water introduced into the slag,on contact with the liquid slag after cooling converts into water vapourwhich diffuses in the slag to obtain a porous structure in the latterwhen it is solidified. Moreover the diffusion of the water in the slagcombined with slow cooling provides the solidified slag with acrystalline or vitreous structure which is stable at ambient temperatureallowing the slag to be utilised. In addition, the economic significanceof the method according to the invention which makes it possible toenhance the economic value of the slag from steelworks furnaces,contrary to prior art, considerably improving the profitability ofsteelworks, will be readily understood.

The rate of cooling and the total period of cooling will be selectedaccording to the phase or phases to be formed in the end product.

Other advantages and characteristics will appear better from thedescription below of a number of variants, given as non-limitativeexamples of the slag treatment method and devices implementing themethod, with reference to the attached drawings where:

FIG. 1 is a schematic representation showing a preliminary treatmentstage of the liquid slag;

FIG. 2 is a schematic partial longitudinal section of a device of theinvention;

FIG. 3 is a schematic partial longitudinal section of a first variant ofthe device of the invention;

FIG. 4 is schematic partial longitudinal section of a second variant ofthe device of the invention; and

FIG. 5 is schematic partial longitudinal section of a last variant ofthe device of the invention.

With reference to FIG. 1, the molten material coming from a steelworksfurnace is conventionally formed of slag 1 and residual steel 2 which ispoured into a tundish 3. The residual steel 2 being denser than the slag1 is deposited on the bottom of the tundish 1 after pouring out, the itis separated from the slag 1 according to a process well known to theexpert, the said residual steel 2 being treated separately.

The slag 1 which is contained in the tundish 3 at a temperature higherthan 1000° C., with reference to FIG. 2, is liquid. The treatment of thesaid slag 1 consists in introducing water, at ambient temperature forinstance, through a lance 4 plunged into the said slag 1. On contactwith the liquid slag 1, the water converts into water vapour 5 whichdiffuses in the slag providing it with agitation and oxidising theelements of the said slag 1 in such a manner that, after it has cooled,the solidified slag has a crystalline or vitrified structure which isstable at ambient temperature as will be seen further on. In addition,it will be noted that as the lance 4 is plunged sufficiently deeply inthe slag 1 the introduction of water into the slag 1 does not cause aviolent disruption of the material.

In order to cool the slag 1, the tundish 3 is placed to the right of acontinuous conveyor 6 carrying moulds 7 of cast iron or steelarticulated in two parts and extending between a leading roller 8 a anda led roller 8 b. The moulds 7 have a thickness which is defined by thequantity of heat to be absorbed in a time interval given by thedimensions of the continuous conveyor 6 and by the forward speed of theconveyor 6. The slag 1 is poured into a pouring chute 9 opening outabove the moulds 7 of the conveyor 6 so as to pour the slag 1 on to themoulds 7 continuously and in a stream. The continuous conveyor 6 advancerate is chosen so that the thickness of the strip of slag 1 is between 3and 5 cm approximately so as to obtain slow cooling of the said slag 1.This slow cooling of the slag 1 makes it possible to avoid instantaneoussolidification and to maintain, notably, the calcium/silicate C₃S and/orC₂S phases of the slag 1 in crystalline form, the crystalline form beingobtained by means of introduction of water into the liquid slag 1 beforeit is cooled as has been described above. In this respect, the watervapour 5 diffused in the slag 1 is trapped by the slag 1 in the courseof cooling obtaining a porous structure of the solidified slag 1.

At the end of the continuous conveyor 6, to the right on FIG. 2, thesolidified slag 1 the porous structure whereof makes it friable orcrumbly or flaky so that it fractures to form particles of slag 1 whichcan be recovered. In a particularly advantageous manner, the particlesare ground, crushed or granulated, then sieved to grade them to obtaingranular materials of different dimensions such as rubble, riddledmaterial etc capable of being used as railway track ballast, or added toaggregate, cement or similar.

In a variation water can be fine sprayed on the solidifying slag 1 closeto the end of the conveyor 6, as indicated by the arrows 11 of FIG. 2,so as to finalise the cooling of the slag 1. Moreover, water can also befine sprayed on the moulds 7 in the course of their return path, asindicated by the arrows 12, in order to cool them.

It is quite obvious that in order to improve the physical and chemicalproperties such as the “hydraulicity” of the solidified slag obtainedaccording to the invention, an additive may advantageously be introducedinto the water and/or the slag 1. The additive may contain one or moreof a number of elements such as Si, Al, Fe, Ca, Cr, O, B, K, F and P.

According to a variant of the treatment device of the invention, shownin FIG. 3, the device comprises a hopper 13 containing a fine granularmaterial 14 which consists, preferably, in fines, i.e. in a granularmaterial of metallurgical slag the mean diameter of which is between 0and 6 mm. The hopper 13 is positioned to the right of the continuousconveyor 6 above the tundish 3 containing the slag 1 and defines meansadapted to pour the granular material on the conveyor 6. The devicecomprises means for spraying water 15 positioned to the right of theconveyor 6 between the hopper 13 and the tundish 3 in order to moistenthe granular material 14. The water is, preferably, sprayed on thegranular material 14 by finely spraying in such a manner that thegranular material is not saturated with water, i.e. that no pool ofwater is formed on the surface of the granular material 14. In the samemanner as above, the liquid slag 1 is poured continuously and in astream through the pouring chute 9 on the granular material 14 moistenedso that on contact with the slag 1, the water converts into water vapourwhich diffuses in the slag 1. Besides the agitation obtained bydiffusion of the water vapour in the slag 1 which makes it possible toobtain, after it has been cooled, a slag with a crystalline structurewhich is stable at ambient temperature, the diffusion of the watervapour entrains the particles of “fines” which combine in the slag toprovide it with enhanced physical and chemical properties.

According to another variant shown in FIG. 4, the device comprises, inthe same manner as above, a continuous conveyor 6 formed of cast iron orsteel moulds 7 articulated in pairs and extending between a leadingroller 8 a and a led roller 8 b and a tundish 3 containing the liquidslag 1 which is poured continuously and in a stream through a pouringspout 9 on the moulds 7. The device comprises, in addition, lances 16positioned along the pouring spout 9 and opening out into the stream ofslag 1 in order to introduce water into the stream, which vaporises oncontact with the slag 1 to provide the agitation necessary to theformation of a crystalline structure of the slag as described above.

It is obvious that the device according to the invention may compriseonly a single lance 16. In another variant shown in FIG. 5, the devicecomprises for the one part a continuous conveyor 6 formed of cast ironor steel screens 17 articulated in pairs with a fine mesh and extendingbetween a leading roller 8 a and a led roller 8 b and for the other parta tundish 3 containing the liquid slag 1 which is poured continuouslyand in a stream through a pouring spout 9 on to the screens 17. Thedevice comprises, in addition, means for spraying 18 water on theinternal face of the screens 17, i.e. on the face opposite the face onwhich the slag 1 is poured, the sprayers 18 being positioned between theupper edge and the lower edge (called return) of the continuous conveyor6 at the level of the pouring point of the slag 1 on the conveyor 6 orslightly below this point. It is worth noting that the water trappedbetween the mesh of the screens 17 converts, on contact with the slag 1,into water vapour which diffuses in the slag before it cools gradually.

It will be observed that the solidified slag obtained according to themethod of the invention consists of a porous solidified slag in whichthe calcium silicate phases are stable and in crystalline form.Moreover, according to the volume of water used in the course of theprocess, the solidified slag may comprise or include hydrated lime.

It is obvious that, according to the selected cooling time of the slag1, a solidified slag will be obtained which has a crystalline structureor a vitrified structure which are both stable at ambient temperature.

Finally, it is obvious that the method according to the invention may beimplemented for blast furnace slag, although the treatment of blastfurnace slag is not necessary taking into account its natural stability,and that the examples which have just been given are only particularillustrations which are not intended to limit the uses of the invention.

1. A method for treating steel slags coming out from steelworks furnacesof the LD type, converter, electric are or similar, being pouredcontinuously onto continuous/endless conveyor formed of moulds and movedin order to gradually cool the liquid slag which is solidifying, whereinsaid method comprises introducing a lance to penetrate the liquid steelslag and thereby introducing and diffusing water into the liquid slagwithout causing an explosion thereof.
 2. A method according to claim 1,comprising introducing at least one lance into a tundish containing theliquid slag before the slag is poured on the conveyor.
 3. A methodaccording to claim 1, further including pouring a fine granular materialfrom a hopper adjacent the conveyor onto the conveyor and spraying wateronto the fine granular material.
 4. A method according to claim 1,further comprising controlling the rate at which the resultant materialcools so that the solidified steel slag has a porous, crystalline orvitrified structure dependent on the rate of cooling, which structure isstable at ambient temperature.
 5. A method according to claim 1,including introducing the lance into the slag flowing into a pouringchute that its inclined downwards from a tundish to the conveyor.
 6. Amethod according to claim 1, including providing the conveyor made fromfine mesh screens arranged in pairs and spraying water on the internalface of the screens.